EFFECT OF PH, IRRADIANCE AND POPULATION-SIZE ON THE TOXICITY OF FURADAN TO 2 SPECIES OF ANABAENA

In two selected nitrogen-fixing cyanobacterial species Anabaena fertil issima and Anabaena variabilis pH, irradiance and different inocula si zes considerably modified the toxic effect of 50 % effective concentra tion (EC 50) dose of the pesticide Furadan (carbofuran 75 DB). Maximum growth and chlorophyll (Chi) a content of A. fertilissima was observe d in the pH range of 8 - 9 and that of A. variabilis at pH 7 - 8, whil e at acidic pH (5 - 6) and at pH above 9 these parameters were conside rably retarded. Toxicity of the EC 50 dose of Furadan was increased fu rther at pH 5 - 6, whereas reduction in the toxicity to the test cyano bacteria was observed at pH 7.8 - 9.0. The experimental organisms grew comparatively better and synthesized higher amount of Chi a at an irr adiance of 12.5 than at 7.5 or 2.5 W m(-2). The toxicity of EC 50 dose of the pesticide gradually decreased with the increasing irradiance. The toxic effect of Furadan was larger when the initial cyanobacterial population concentration was low and vice versa.